Article pubs.acs.org/EF
Pyrolysis and Char Characterization of Refuse-Derived Fuel Components Rita Barros Silva,† Susete Martins-Dias,† Cristina Arnal,‡ María U. Alzueta,‡ and Mário Costa*,§ †
Institute for Biotechnology and Bioengineering, CERENA and §IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal ‡ Aragón Institute of Engineering Research, Chemical and Environmental Engineering Department, University of Zaragoza, Zaragoza, Spain ABSTRACT: This work concentrates on the pyrolysis and char characterization of a refuse-derived fuel (RDF), produced from municipal solid waste, and five of its components, namely, nonpackaging paper, food, textile, low-density polyethylene film, LDPE, and packaging polypropylene wastes. Specifically, this work characterizes physically and chemically the materials, examines the pyrolysis and oxidation patterns of the RDF and its components using thermogravimetric analysis (TGA) and a quartz reactor (QR), respectively, evaluates the reactivity toward oxygen of selected RDF chars at 650 °C in the QR, and characterizes the trace organics emission during the pyrolysis and chars oxidation in the QR by means of gas chromatography coupled with mass spectrometry. The main conclusions of this study are as follows: (1) the pyrolysis and oxidation patterns indicate the presence of low-stability organic components, lignocellulosic materials, and plastics but with the maximum rates occurring at lower temperatures than those reported in the literature; (2) the reactivity toward oxygen of chars at 650 °C revealed that the food waste and the LDPE film waste chars are among the less reactive chars, while the textiles waste and the packaging PP waste chars are among the more reactive chars; the RDF char reactivity resembles mostly that of the nonpackaging paper waste char; (3) analyses of the trace organic elements released reflect the transversal contamination of the different materials that compose RDF, derived from its mixed origin. During both pyrolysis and char combustion, phthalates were the most relevant compounds encountered.
1. INTRODUCTION Refuse-derived fuels (RDF) are alternative solid fuels prepared from nonhazardous waste for application in energy-intensive industries, such as electric power or cement production ones. RDF can be composed of many materials with different characteristics, including plastics, paper, cardboard, and textiles, which confer a high calorific content to the RDF.1 Materials such as paper, cardboard, and textiles are also regarded as biomass owing to their biogenic origin,2 and thus, some part of the RDF is considered renewable and CO2 neutral. RDF is also interesting due to its local availability.3 Industrial waste flows of nonrecyclable paper and plastics, among others, are being shredded and mixed to produce RDF. Likewise, municipal solid waste (MSW) contains a fraction between 20 and 40 wt %,1 often referred to as the high calorific fraction (HCF), which is currently being separated and increasingly used as raw material for RDF production. In brief, it consists of a mixture of the same types of combustible materials found in industrial wastes, like plastics, cardboard, or textiles, but abounding heterogeneously and often impregnated with food and garden scrap. RDF has a high volatile content,4,5 and its burning profile, obtained from thermogravimetric analysis (TGA), shows that the devolatilization and volatiles combustion phases correspond to the most significant and rapid mass loss stages.5,6 Moreover, the oxygen concentration does not affect the devolatilization pattern, even though shifts of the burning profile to lower temperatures can be observed.5 Thus, data of interest regarding the volatiles behavior may be obtained from pyrolysis studies of © XXXX American Chemical Society
RDF and/or its main components. The literature covers a wide range of materials such as putrescible organics,7 lignocellulosics,7,8 plastics,7,9,10 and waste mixtures.5,7 Heikkinen et al.7 investigated the devolatilization pattern of a broad range of materials through TGA and concluded that single components could be grouped into classes such as lowstability organic components, lignocellulosic materials, and plastics. Additionally, they found that it was difficult or impossible to distinguish between materials belonging to the same class based on TGA data only. Results for pyrolysis of RDF5 allow distinguishing mainly the two latter groups of materials: biomass/paper materials (lignocellulosic) and plastics. While both types of materials devolatilize in a single phase, this process happens at different temperature. First, paper and other biomass materials degrade similarly between 300 and 400 °C, reflecting the presence of hemicellulose, cellulose, and lignin. Maximum pyrolysis rates are achieved in the range of 365−375 °C, which corresponds mainly to cellulose. Next, plastics devolatilize in a narrow temperature range (±50 °C) according to the type of plastic9 but normally within the interval of 400−550 °C. As a result, some authors model the pyrolysis of RDF following a first-order reaction11 or a combination of independent, first-order, parallel reactions describing the decomposition of the two/three main groups of components described above.5 Received: September 8, 2014 Revised: February 3, 2015
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DOI: 10.1021/ef502011f Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Quartz fixed bed reactor used for the pyrolysis and combustion tests.
ethylene film, LDPE, and packaging polypropylene (PP) wastes. Specifically, this work characterizes physically and chemically the materials, examines the pyrolysis and oxidation patterns of the RDF and its components using thermogravimetric analysis (TGA) and a quartz reactor (QR), respectively, evaluates the reactivity toward oxygen of selected RDF chars at 650 °C in the QR, and characterizes the trace organics emission during the pyrolysis and chars oxidation in the QR by means of gas chromatography coupled with mass spectrometry (GCMS).
For additional and more extensive characterization of the RDF volatiles release, Hilber et al.4 used TGA in an inert atmosphere combined with other well-established analytical procedures in order to study energy and elements (C, H, N) partitioning between volatiles and char as well as its mass releasing profile. The authors found that in a RDF derived from MSW, mass, energy, and carbon losses had very similar profiles. Results from combustion tests in a drop tube furnace confirmed these findings.12 According to Hilber et al.,4 around 60 wt % of the carbon in the RDF is released at heating temperatures below 400 °C, probably associated with the abundance of biomass and paper materials. Further 20 wt % is lost on increasing the temperature up to 500 °C. Moreover, for biomass and paper/cardboard materials a significant amount of energy was found to stay in the char, despite their high-volatile contents. A possible explanation is that the volatiles released consist of hydrogenbased components, while carbon stays in the char phase. More recently, Montané et al.13 used a similar approach and concluded that energy and carbon are not evenly released along with the volatiles. The volatiles released at low temperatures contain less energy than the remaining char. At high temperatures volatiles comprise roughly the same energy or slightly more, due to the plastics. Carbon behaved similarly, probably for the same reasonthe main componentsstating the importance of carbon for energy release after a first period where most of the hydrogen is released along with volatiles.4,13 The literature reveals that there is a lack of information on the combustion of chars from RDF. Some information on lignocellulosic materials can be gathered from studies focusing on the reactivity of chars from forest biomass,14 studied at relatively high temperatures (900 °C). Recently, a study15 was published focusing on cellulosic-based chars, analyzing the reactivity at 500 °C of materials representing the cellulosic portion of wastes (wood chips, newspaper, and glossy paper). In addition to the characteristics of RDF pyrolysis, information on the properties and reactivity of chars from RDF is essential to fully understand the RDF combustion fundamentals and thereby to improve the performance of combustion systems. In light of the scarce information available in the literature, this work concentrates on the pyrolysis and char characterization of a refuse-derived fuel (RDF), produced from municipal solid waste (MSW), and five of its components, namely, nonpackaging paper, food, textile, low-density poly-
2. EXPERIMENTAL SECTION 2.1. Fuels. In this study six different materials were evaluated, namely, a RDF produced from the HCF of a MSW and five HCF components: nonpackaging paper waste, food waste, textiles waste, LDPE film waste, and packaging PP waste. The HCF corresponds to the fraction rejected during the mechanical pretreatment of unsorted MSW by double-trommel screening (120 and 80 mm) followed by metal magnetic removal. The fraction below 80 mm is mainly composed by putrescible materials and the oversized fraction by recyclables. The HCF was further processed into a fluff RDF (∼30 mm) through serial particle size reduction steps including additional removal of metals and ballistic separation of inert materials. The RDF and HCF components were sampled through representative procedures, by collection of masses ranging from 1 to 5 kg depending upon the material and its initial particle size. RDF sampling was made by random collection of increments (>10) from the stockpiled material at the end of the production process. The HCF components were sampled by fractional shovelling.16 Prior to further processing all samples were dried at 105 °C. Laboratory samples preparation aimed to avoid segregation and increase the sample representativity due mostly to particles size heterogeneity. Samples followed a sequence ground to a particle size of 1 mm (Retsch SM 2000 cutting mill sequentially operated with 10, 4, and 1 mm sieving meshes), alternately with mass reduction steps by mechanical division with a rotary divider (Retsch PT100 with a six divisions head). Some materials had to be hand cut (>30 mm) previously. Cryogenic conditions were used for hardening some of the materials by addition of liquid nitrogen before and during grinding. Next, samples of 1−2 g obtained using the rotary divider went an extra milling step using a cryogenic mill (Spex 6770). The output was a homogeneous powder like material. For all fuels ultimate analyses, ash content, and gross calorific value (GCV) were performed in accordance to standard procedures for RDF (EN 15407, EN 15403, EN15400) at least in triplicate. 2.2. Pyrolysis and Oxidation Patterns. The pyrolysis pattern of the fuels was assessed using a TGA (Netzsch STA 449 F3). Dry B
DOI: 10.1021/ef502011f Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels samples of 16−24 mg were heated from 25 to 650 °C at a rate of 10 °C min−1 in the presence of a constant flow rate of 100 mL min−1 of N2. Total mass was registered every 6 s. The pyrolysis profile was expressed in terms of the pattern of total mass in the course of heating. The oxidation pattern of the fuels was assessed in a QR consisting of a vertical tube with an outer diameter of 15 mm and a total length of 550 mm, as shown in Figure 1.14 For each experiment, dry samples of 10−12 mg of the described RDF and waste components were mixed with 300 mg of silica sand (
